In an inverter main circuit provided in an inverter, which is configured by a bridge-connected semiconductor device unit, a switching loss is generated in the semiconductor device unit due to a switching control by a Pulse Width Modulation (PWM). To prevent the semiconductor device from being breakdown by the heat resulting from the loss, a cooler such as a radiating fin or a fan is arranged in the inverter main circuit. In consideration of downsizing for enhancing the value of an inverter product, downsizing of the cooler is needed. However, to achieve the downsizing, decreasing of the switching loss is first required.
Examples of conventional control techniques for decreasing a switching loss include the following techniques.
(1) Decreasing a Carrier Wave Frequency (Switching Frequency)
(2) Improving a Modulation Technique
The technique (1) has such problems that when the carrier wave frequency is lowered too far, modulation accuracy is degraded and a noise level becomes greater, and thus there is a limit to the decrease.
Meanwhile, examples of the technique (2) include two-phase modulation systems by Patent Document 1 and Nonpatent Literature 1 described below. The two-phase modulation system is characterized in that in a switching control of U, V, and W-phases, one of these phases is not switch-controlled but the remaining two phases only are switch-controlled to generate a required modulation wave.
Nonpatent Literature 1 shows an operation waveform of a line-to-line voltage control PWM system, which is one technique of the two-phase modulation systems (see FIG. 6.3.24 in Nonpatent Literature 1). The line-to-line voltage control PWM system utilizes a fact that, when a three-phase AC device is used as a load, there is flexibility in selection of a phase voltage if the line-to-line voltage can be maintained. There are various modulation waves generated by this system.
For example, FIG. 6.3.24 of Nonpatent Literature 1 depicts a modulation waveform generated by arranging a switching dwell period of a ⅓ cycle for each of U, V, and W-phases in turn while the remaining two phases are switch-controlled in the dwell period.
According to the modulation waveform shown in the Nonpatent Literature 1, the switching dwell period is arranged for each ⅓ cycle (every 120 degrees), and thus, when the same carrier wave condition is provided, the number of switchings can be decreased to approximately ⅔ as compared to the normal three-phase PWM system. That is, it is advantageous that the switching loss can be decreased to approximately ⅔, and thus downsizing of the cooler of the main circuit and downsizing of the whole apparatus can be achieved.
In the modulation waveform generated by the line-to-line voltage control PWM system, at a timing of each end of the switching dwell periods arranged for every 120 degrees, the phases in which the switching is dwelled are changed over. Accordingly, a discontinuous inflection point is generated in the modulation wave of each phase, and unnecessary switchings are increased. On the other hand, in the modulation waveform shown in Nonpatent Literature 1, a changeover of phases in which the switching is dwelled is performed with a minimum value or a maximum value of the carrier wave to inhibit an increase in the unnecessary switchings.
Meanwhile, there are various forms of the two-phase modulation, and Patent Document 1 shows an example of a modulation wave in which the switching dwell periods are arranged for every 60 degrees. As compared to the system in which the dwell periods are changed over for every 120 degrees, the system in which the dwell periods are changed over for every 60 degrees can generate a modulation wave having positive and negative symmetry, resulting in a better loss balance in upper and lower devices configuring the inverter main circuit and enabling to more easily design a cooler.    Patent Document 1: Japanese Patent Application Laid-open No. 2001-352790    Nonpatent Literature 1: Semiconductor power converting circuit (Editor and Publisher: The Institute of Electrical Engineers of Japan, Selling agency: Ohmsha), pp. 130